1. Field of the Invention
The invention relates to the machining of ophthalmic lenses, i.e. to the set of operations aimed at giving the rim of the lens a predetermined shape corresponding as precisely as possible to that of the ring of a spectacle frame in which the lens is intended to be mounted.
2. Description of the Related Art
It will be recollected that a lens, mounted on a support shaft, subjected firstly to a trueing operation in which it is ground along its rim until the latter has roughly—i.e., generally, with allowance—the predetermined shape, then one or more finishing operations consisting of refining the shape of the rim, and, optionally, giving it a particular structural configuration, for example by machining a bevel or, on the other hand, a groove on the rim.
As soon as these operations have been carried out, the lens is removed from its support shaft to be mounted in the ring of the frame. However, it may then be noticed that the lens is not perfectly matched to said ring, which means that an additional reworking operation is required, which consists of removing again a certain quantity of material from the lens along its rim in order to match the ring of the frame.
Generally, when the lens requires only minor reworkings, such a reworking operation is carried out manually by the optician.
However, such a manual reworking operation involves, for the optician, constant trips between the grinding machine and the frame to check if the lens corresponds to the ring of the frame, which is tedious.
Moreover, such a manual reworking is, by nature, imprecise, and it happens that defects not visible to the naked eye are detrimental to the firm grip of the lens in the ring of the frame.
The imprecision of this manual reworking can also lead to the lens being discarded if it has incorrectable defects.
For this reason, the trend is towards the automation of the reworking to which lenses must be subjected. However, for this automation to be possible, it is necessary to know the shape of the contour which was given to the rim during the machining operations, whether trueing or finishing was involved.
Moreover, the lens can be removed from its support shaft, also, between the trueing operation and the subsequent finishing operation, whether to carry out a first verification of the correspondence between the shape of the lens and that of the ring of the frame or, if necessary, to change the machining device.
In either of the cases which will be presented, removing the lens from its support shaft can cause the loss of the three-dimensional reference mark linked to the lens and used to give the predetermined shape to the rim of the lens during machining.
Solutions have been proposed to preserve this reference mark when the lens is separated from its support shaft.
Thus, certain support shafts have a detachable part at the end, commonly called “gland or holding means” which, positioned and oriented in a precise manner on one of the faces of the lens, is fixed on the latter, generally by means of an adhesive.
Such a “gland or holding means”, which has correction means for mounting at the end of the support shaft certainly allows the preservation of the reference mark linked to the lens.
However, the fitting of this “gland or holding means” on the lens requires numerous manipulations such as the definition and the location of a point on the lens for fixing the “gland or holding means”. Generally, the fitting of the “gland or holding means” is carried out elsewhere by means of a costly special machine.